Root exudates include important chelating compounds and can change the rhizosphere pH by several units. These changes are essential for nutrient uptake and can also alter solubility of soil contaminants and increase plant uptake. Mild root-zone stress may increase exudation and more severe stress can damage membranes and increase root turnover, all of which increase root-zone carbon. Increased carbon from this rhizodeposition can increase microbial activity, which might help degrade contaminants. We studied the effect of three types of stress on root exudation of crested wheatgrass (Agropyron cristatum): low K+, drought, and flooding. These stresses were compared to two types of controls: 100% NO3- and high NH4+:NO3- ratio. We developed an improved axenic system to keep plants microbe-free for 70 days while analyzing exudates for total organic carbon (TOC) and organic acids. Axenic conditions were confirmed by plate counts of the leachate and microscopic observations of the leachate and roots. Optimal conditions for plant growth were maintained by monitoring temperature, light, humidity, water, O2, CO2, nutrient availability, and root-zone pH. Plants were grown in Ottawa sand that was layered by size to optimize water availability. Total organic carbon released over the 70-day growth period in mg per gram dry plant was 2.6 in the control, 2.3 in the NH4+ treatment, 3.7 in the flood and K+ stress treatments, and 4.4 in the drought treatment, which was the only treatment significantly higher than controls (p = 0.05). TOC and organic acid levels in the exudates peaked before the end of the study. The peak TOC levels, expressed as mg TOC per gram new dry plant mass, were 1.9 in the control, 3.0 in the NH4+ treatment, 2.9 in the flood, and 5.8 in the drought and K+ stress treatments. Organic acids were measured by gas chromatography-mass spectrometry (GC-MS). Malic acid was the predominant organic acid, and accounted for the majority of the TOC in the drought treatment. Oxalic, succinic, fumaric, and malonic acids accounted for less than 10% of the TOC. These data indicate that stress may enhance phytoremediation by changing root-zone exudate composition.
| Identifer | oai:union.ndltd.org:UTAHS/oai:digitalcommons.usu.edu:etd-6719 |
| Date | 01 May 2003 |
| Creators | Henry, Amelia |
| Publisher | DigitalCommons@USU |
| Source Sets | Utah State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | All Graduate Theses and Dissertations |
| Rights | Copyright for this work is held by the author. Transmission or reproduction of materials protected by copyright beyond that allowed by fair use requires the written permission of the copyright owners. Works not in the public domain cannot be commercially exploited without permission of the copyright owner. Responsibility for any use rests exclusively with the user. For more information contact digitalcommons@usu.edu. |
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